3 1/2 cups unbleached all-purpose flour
2 teaspoons salt
2 1/2 teaspoons active dry yeast
1 1/3 cups warm water, about 110 degrees
4 tablespoons olive oil
4 tablespoons olive oil, plus more for sprinkling the top of the dough before baking
1 10 x 15-inch jellyroll pan brushed with 3 tablespoons olive oil
For the dough, stir together the flour and salt in a large mixing bowl. Whisk the yeast into the water and then whisk in the oil. Use a large rubber spatula to mix the liquid into the flour. Continue stirring vigorously until the dough is fairly smooth and there are no longer any dry patches. Cover the bowl with plastic wrap and let the dough rise at room temperature until it doubles in bulk, about 1 hour.
While the dough is rising, cook the onions.
Put the 4 tablespoons oil and onions in a wide sauté pan and place over medium heat. Season very lightly with salt (the anchovies and olives are both salty) and grind over about 5 or 6 turns of the peppermill. Once the onions start to sizzle, decrease the heat to low and cook until the onions are very soft and translucent, about 20 to 30 minutes. Scrape the onions into a bowl and set aside to cool.
When the dough has risen, scrape it into the prepared pan in one piece. Lightly oil your hands and press the dough into the pan to cover it completely. If the dough resists, wait 5 minutes, then pull the dough into the corners of the pan. Cover the dough with oiled plastic wrap and set it aside until it puffs a little, about 20 to 30 minutes. (It does not need to double in bulk.)
As soon as you cover the dough, set a rack in the lowest level of the oven and preheat to 400 degrees. When the dough has puffed, use a fingertip to indent it at 1 1/2-inch intervals all over the surface, pressing firmly but without piercing the dough through to the bottom of the pan.
Drizzle the topping with 1 or 2 tablespoons of oil and sprinkle with the Kosher or coarse salt.
Bake the focaccia until it is well risen and the bottom is well colored, about 30 minutes. Lift a corner of the focaccia with a large metal spatula to check that the bottom is well done.
Use a wide metal spatula to slide the focaccia from the pan to a rack to cool.